Systems and methods for controlling power in an electrosurgical probe

ABSTRACT

Systems and methods for controlling the power supplied to an electrosurgical probe are disclosed. The systems and methods may be used to monitor electrode-tissue contact, adjust power in response to a loss of contact, and apply power in such a manner that charring, coagulum formation and tissue popping are less likely to occur.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to structures for positioningone or more diagnostic or therapeutic elements within the body and, moreparticularly, to power control systems for use with the same.

2. Description of the Related Art

There are many instances where diagnostic and therapeutic elements mustbe inserted into the body. One instance involves the treatment ofcardiac conditions such as atrial fibrillation and atrial flutter whichlead to an unpleasant, irregular heart beat, called arrhythmia.

Normal sinus rhythm of the heart begins with the sinoatrial node (or "SAnode") generating an electrical impulse. The impulse usually propagatesuniformly across the right and left atria and the atrial septum to theatrioventricular node (or "AV node"). This propagation causes the atriato contract in an organized way to transport blood from the atria to theventricles, and to provide timed stimulation of the ventricles. The AVnode regulates the propagation delay to the atrioventricular bundle (or"HIS" bundle). This coordination of the electrical activity of the heartcauses atrial systole during ventricular diastole. This, in turn,improves the mechanical function of the heart. Atrial fibrillationoccurs when anatomical obstacles in the heart disrupt the normallyuniform propagation of electrical impulses in the atria. Theseanatomical obstacles (called "conduction blocks") can cause theelectrical impulse to degenerate into several circular wavelets thatcirculate about the obstacles. These wavelets, called "reentrycircuits," disrupt the normally uniform activation of the left and rightatria.

Because of a loss of atrioventricular synchrony, the people who sufferfrom atrial fibrillation and flutter also suffer the consequences ofimpaired hemodynamics and loss of cardiac efficiency. They are also atgreater risk of stroke and other thromboembolic complications because ofloss of effective contraction and atrial stasis.

Although pharmacological treatment is available for atrial fibrillationand flutter, the treatment is far from perfect. Many believe that theonly way to treat the detrimental results of atrial fibrillation andflutter is to actively interrupt all of the potential pathways foratrial reentry circuits.

One surgical method of treating atrial fibrillation by interruptingpathways for reentry circuits is the so-called "maze procedure" whichrelies on a prescribed pattern of incisions to anatomically create aconvoluted path, or maze, for electrical propagation within the left andright atria. The incisions direct the electrical impulse from the SAnode along a specified route through all regions of both atria, causinguniform contraction required for normal atrial transport function. Theincisions finally direct the impulse to the AV node to activate theventricles, restoring normal atrioventricular synchrony. The incisionsare also carefully placed to interrupt the conduction routes of the mostcommon reentry circuits. The maze procedure has been found veryeffective in curing atrial fibrillation. However, the maze procedure istechnically difficult to do. It also requires open heart surgery and isvery expensive. Thus, despite its considerable clinical success, only afew maze procedures are done each year.

More recently, maze-like procedures have been developed utilizingcatheters and probes which can form lesions on the endocardium toeffectively create a maze for electrical conduction in a predeterminedpath. Exemplary catheters are disclosed in commonly assigned U.S. Pat.No. 5,582,609. Exemplary surgical soft tissue coagulation probesemploying a relatively shorter and stiffer shaft than a typical catheterare disclosed in commonly assigned U.S. patent application Ser. No.08/949,117, filed Oct. 10, 1997, and U.S. patent application Ser. No.09/072,835, filed May 5, 1998, both of which are incorporated byreference. Such probes may, for example, be used to treat atrialfibrillation in procedures wherein access to the heart is obtained byway of a thoracostomy, thoracotomy or median sternotomy.

Typically, the lesions are formed by ablating tissue with an electrodecarried by the catheter or ablation probe. Electromagnetic radiofrequency ("RF") energy applied by the electrode heats, and eventuallykills (or "ablates") the tissue to form a lesion. During the ablation ofsoft tissue (i.e. tissue other than blood, bone and connective tissue),tissue coagulation occurs and it is the coagulation that kills thetissue. Thus, references to the ablation of soft tissue are necessarilyreferences to soft tissue coagulation. "Tissue coagulation" is theprocess of denaturing proteins in tissue and heating the fluid withinthe tissue cell membranes which causes it to jell, thereby killing thetissue.

A primary goal of many soft tissue coagulation procedures is to createcontiguous lesions (often long, curvilinear lesions) withoutover-heating tissue and causing coagulum and charring. Soft tissuecoagulation occurs at 50° C., while over-heating occurs at 100° C. Aproblem in the related art is the issue of rapid turning on and offpower when a coagulation electrode loses contact with tissue. Tissue incontact with a coagulation electrode acts as a load to the power circuitpowering the electrode, usually an RF power circuit. When thecoagulation electrode is pulled away from tissue or efficacious contactis lost, the load is removed, and the voltage output of the powercircuit may change. Voltage may rise suddenly, which can cause problemswhen the electrode is reintroduced into contact with tissue, such asarcing or charring. As a safety consideration, the circuit inconventional systems is powered off for a predetermined period byturning off the power to the RF coagulation electrode when contact islost.

However, the inventors herein have determined that powering a circuitcompletely off can result in a number of problems. For example, abruptpowering on of a coagulation electrode can char tissue if the voltagerise is too rapid. Additionally, powering a circuit completely offintroduces the delay associated with powering the circuit back on intothe procedure. Not only is this delay inconvenient, it can also bedetrimental to the patient, especially since the loss of contact canhappen many times during a procedure. For example, soft tissuecoagulation probes can be used to perform a maze procedure during amitral valve replacement, which requires cardiopulmonary bypass. Thelonger the patient is on bypass, the greater the likelihood of morbidityand mortality. Consequently, there is a need to quickly recover from aloss of electrode-tissue contact, without completely shutting off thepower supply.

Another problem identified by the present inventors has been verifyingthat tissue is in contact with a coagulation electrode prior to orduring a surgical or catheter-based procedure, which is generally termedelectrode contact verification. This is a problem pervasive throughoutall surgery being performed remotely, especially when direct visualline-of-sight is not present. The use of fluoroscopic techniques issomewhat inaccurate, and requires the use of human feedback.Accordingly, a need exists for an automated control system for electrodecontact verification, and optionally with visual and/or audio feedbackwhen there is loss of contact between the electrode and tissue.

Yet another problem identified by the present inventors is associatedwith tissue treatment efficacy when coagulating tissue. Specifically,because different tissues in the human body and between patients absorbenergy at different rates, it is difficult to ascertain when propertissue coagulation has been completed. Heretofore, ad hoc techniqueshave been used to determine when the soft tissue coagulation process hasbeen completed. One technique is visual inspection. Another is applyingcoagulation energy for a predetermined period based on an estimate ofthe amount of time required to produce a therapeutic lesion. Suchtechniques are not always as reliable as desired. Thus, there is a needfor accurately determining when tissue has been properly coagulated, sothat coagulation may be automatically stopped.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide anapparatus for positioning an operative element (such as a coagulationelectrode) within the body that avoids the aforementioned problems.Other operative elements include lumens for chemical or cryogeniccoagulation, laser arrays, ultrasonic transducers, microwave electrodes,and D.C. hot wires.

In accordance with one advantageous aspect of a present invention, apower control system is provided which, in response to an indicationthat mechanical or efficacious contact between an electrode and tissuehas been lost, will merely reduce the power to the electrode, ratherthan completely shut it off. In one embodiment, this is accomplished byramping down the amplitude of a RF waveform supplied to the electrode toa lower level. Adjustment of the RF signal amplitude need not involveshutting off the RF power supply.

One exemplary method of determining when contact is lost involves theuse of impedance measurements. Here, the impedance is measured andcompared to an expected impedance. When the impedance is greater thanthe expected impedance, the RF source is driven to a relatively lowvoltage level, such as 5V, that allows safe continued operation andimpedance measurement. When the measured impedance is less than theexpected impedance, the RF source ramps up to the levels needed to reachthe set temperature.

There are a number of advantages associated with such a system. Forexample, merely reducing voltage, as opposed to shutting it offcompletely, increases the speed at which probe-based maze proceduresproceed by reducing the down time resulting from a loss of mechanical orefficacious electrode-tissue contact. In addition, when tissue contactresumes, voltage is ramped back up. As such, tissue damage due to anabrupt voltage rise is avoided.

In accordance with another present invention, a control system isprovided which verifies electrode-tissue contact. Preferably, data arecollected relating to variables that can be used by a processor forelectrode contact verification. The data, which are sampled by thecontroller before and/or during the soft tissue coagulation process, maybe temperature or tissue impedance data. For example, a rise intemperature over a predetermined period of time usually means that theelectrode is in contact with tissue and is heating tissue rather thanblood. Such a temperature rise may be measured prior to coagulation byapplying a small amount of energy (less than that required forcoagulation) to the tissue to verify contact. Conversely, a drop-off intemperature during coagulation may mean contact has been lost. In thecase of impedance, a flat profile of impedance over frequency indicatesthat there is no tissue-electrode contact.

When it is determined from the data that efficacious electrode-tissuecontact has been lost, the processor instructs the console to sound avisual or audio alarm. Voltage may also be ramped down as describedabove. In addition to the advantages related to power control andaudio/visual feedback, this invention also reduces reliance onfluoroscopic techniques.

In accordance with another present invention, a control system isprovided which determines when the soft tissue coagulation process iscompleted. In one embodiment, tissue impedance measurements are used todetermine efficacious lesion formation. A change in the impedance versusfrequency curve, from a sloping curve to a flat curve, indicates thattissue coagulation is completed. Temperature can also be used.Specifically, for a given coagulation energy level and time period, apredetermined temperature profile over time indicates that a lesion hasbeen formed. As a result, coagulation procedures, especially thoseinvolving the formation of multiple therapeutic lesions, may beperformed more efficiently.

In accordance with still another present invention, a control system isprovided which brings the temperature at the electrode to a temperaturethat is less than the maximum set temperature, maintains thattemperature at the electrode at this temperature for a predeterminedperiod, and then increases the temperature at the electrode to the settemperature. In a preferred embodiment, the temperature at which tissueis maintained prior to ramping up may only be sufficient to create atransmural lesion in a relatively thin anatomical structure, while theset temperature is sufficient to create a transmural lesion in arelatively thick structure. Such a control system provides a number ofimportant benefits. For example, in those instances where the tissuestructure turns out to be relatively thin, a transmural lesion may becompleted (and power delivery stopped) before the temperature reachesthe set temperature because lower temperatures will be automaticallyused prior to reaching the set temperature. In other words, the systemautomatically attempts to form a lesion at a lower temperature beforeramping up to the higher temperature. As many lesions will be formed atthe lower temperature, coagulation procedures performed using thepresent control system are less likely to cause tissue charring andcoagulum formation than procedures performed with conventional controlsystems.

The variable temperature set point system described in the precedingparagraph is also useful in epicardial applications where electrodes areplaced on the epicardial surface of a heart chamber. Here, blood flowwithin the heart chamber produces a convective cooling effect on theheart surface and makes the creation of transmural lesions from theepicardial surface more difficult. As a result, higher temperatures(measured at the electrodes) or increased energy delivery duration isrequired to create a transmural lesion. Ramping the temperature to atemperature that is less than the maximum set temperature andmaintaining that temperature for a predetermined period causesdesiccation of the epicardial tissue and improves electrode/tissuecontact. Then, when the temperature is increased to the maximum settemperature, tissue vaporization is less likely because the tissue isalready desiccated. Conversely, when the temperature of tissue that hasnot been desiccated is suddenly increased from body temperature to themaximum set temperature required to make transmural lesions on theepicardial surface, vaporization commonly occurs. This can lead toperforation of the myocardium or the dislodgment of tissue.

Tissue coagulation depth can also be controlled by the varying thelength of RF delivery. Longer RF applications usually produce deepertissue coagulation.

In accordance with still another present invention, an interface isprovided which audibly or visually indicates the status of the variousaspects of the system such as, for example, ablation power, temperature,tissue/electrode contact, tissue impedance, time elapsed, type ofelectrode, and type of probe. In a preferred embodiment, the console maybe driven by software that is modular and upgradeable to allow for newparameters to be displayed and monitored.

The above described and many other features and attendant advantages ofthe present invention will become apparent as the invention becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Detailed description of preferred embodiments of the invention will bemade with reference to the accompanying drawings.

FIG. 1 is a schematic circuit block diagram of one embodiment of thepresent invention.

FIG. 2 is a perspective view of a tissue coagulation system inaccordance with one embodiment of the present invention.

FIGS. 3 and 4 are schematic views of a system for controlling theapplication of ablating energy to multiple electrodes using multipletemperature sensors.

FIG. 5 is a schematic flow chart showing an implementation of thetemperature feedback controller shown in FIGS. 3 and 4, using individualamplitude control with collective duty cycle control.

FIG. 6 is a schematic flow chart showing an implementation of thetemperature feedback controller shown in FIGS. 3 and 4, using individualduty cycle control with collective amplitude control.

FIG. 7 is a schematic flow chart showing an implementation of thetemperature feedback controller shown in FIGS. 3 and 4, usingtemperature control with hysteresis.

FIG. 8 is a schematic flow chart showing an implementation of thetemperature feedback controller shown in FIGS. 3 and 4, using variableamplitude and differential temperature disabling.

FIG. 9 is a schematic flow chart showing an implementation of thetemperature feedback controller shown in FIGS. 3 and 4, usingdifferential temperature disabling.

FIG. 10 is a schematic view of a neural network predictor, whichreceives as input the temperatures sensed by multiple sensing elementsat a given electrode region and outputs a predicted temperature of thehottest tissue region.

FIGS. 11(a), 11(b) and 11(c) are graphs illustrating impedance versusfrequency.

FIG. 11(d) is a schematic diagram of an impedance measurement technique.

FIG. 12 is a flowchart illustrating the steps for software operation ofcertain components in accordance with one embodiment of the presentinvention.

FIG. 13 is graph showing a variable temperature setpoint in accordancewith a preferred embodiment of a present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of the best presently known modeof carrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of illustrating thegeneral principles of the invention.

The detailed description of the preferred embodiments is organized asfollows:

I. Overview of the System

II. Electrosurgical Probe

III. Power Control

IV. Monitoring Tissue Contact, Coagulation Efficacy and Tissue Type

V. Variable Temperature Set Point

VI. User Interface System

The section titles and overall organization of the present detaileddescription are for the purpose of convenience only and are not intendedto limit the present invention.

I. Overview of the System

A schematic block circuit diagram of one preferred embodiment of thepresent invention is shown in FIG. 1. The exemplary system 100 employs aprocessor 102 to command the overall system. In the preferredembodiment, the processor is a microprocessor. However, any suitablecontroller, microcomputer, hardwired or preconfigured dedicatedprocessor, or ASIC may be used for command and control. The processor102 can have a separate and isolated power supply for safety purposes.

In the illustrated embodiment, the processor 102 is connected to an RFpower supply controller 104, which regulates an AC power supply 106 thatsupplies RF power. RF power is preferably produced as a continuoussinusoidal waveform. However, other waveforms such as non-sinusoidal orpulsed can be used. RF power is received by a one-to-many powerswitching unit 112 for transmitting RF power to one or more electrodeleads 121 that are attached to electrodes 115 on a probe 108. Theselection of which electrodes 115 are supplied with RF energy by thepower switching unit 112 is under the control of processor 102.

The processor 102 is also connected through suitable I/O lines to aconsole (or "user interface") 103, which may be part of a single unitthat also includes the processor and power supply/control apparatus (seeFIG. 2) or, alternatively, a separate device. The console 103 preferablyincludes keys, LED readouts, indicator lights, and audio and visualalarms. One example of a suitable user interface is illustrated in FIG.2. A plurality of ports located on the console 103, and operablyconnected to the processor 102, may be used to connect the system 100with other devices that may be used in a medical procedure.

The probe 108 may be any instrument capable of applying electro-magneticenergy, especially in the RF frequency range, including catheters, butpreferably is a surgical probe such as those disclosed in U.S. patentapplications Ser. No. 08/949,117, filed Oct. 10, 1997, and Ser. No.09/072,835, filed May 5, 1998, both incorporated herein by reference.Ablation probe 108 has a distal end 111 and a proximal end 113 attachedto a handle (shown in FIG. 2). Initiation of RF power to the electrodeson the probe can be accomplished through the use of a footswitch, abutton on the probe handle, or a remote control device (as shown in FIG.2 and discussed in Section II).

A number of sensor elements 120 reside on the probe 108 to measure dataparameters relevant to the tissue being treated, such as tissuetemperature. The sensors 120 are spaced apart from one another and arelocated on or near the electrodes 115. The sensors may be in the form oftransducers, thermocouples, thermistors, or other collection electrodes.Sensors 120 receive data in analog form, which is converted to digitalform. The data signals collected from sensors 120 are interfaced, signalconditioned, calibrated, range checked, converted from analog to digitaland multiplexed at a signal conditioner, A/D converter and multiplexcircuit unit 118, and received by processor 102 via I/O data signal line130. The leads 121, 122 shown outside the probe 108 in FIG. 1 would inactual manufacture reside within the probe. For enhanced signalreconstruction, the processor 102 samples the sensors 120 to collectdata at a frequency at least greater than the Nyquist samplingfrequency.

With respect to the use of temperature sensors, details of a temperaturecontroller and neural network for predicting temperature are describedin Section III below. Once the coagulation procedure has started, thetemperature sensors may be used to monitor the temperature setpoint,which is typically in a range from 50° to 100° C., and maintain thetissue temperature at the setpoint by increasing and decreasing power tothe electrodes as needed. As described in detail below, data from thetemperature sensors may also determine electrode-tissue contact and whena coagulation procedure is complete.

Turning to impedance, a flattening of the impedance versus frequencycurve is either an indication of a lack of tissue contact, or anindication that the tissue is coagulated. Dedicated impedance electrodesmay be provided on the probe. However, impedance is preferably monitoredby simply measuring the voltage and current at different frequenciesthrough the coagulating electrodes and dividing the measured voltage bythe measured current. The level of current supplied to each electrodemay also be used as a control parameter. These aspects of the exemplaryembodiment are described in Sections IV and V below and in U.S. Pat. No.5,577,509, which is incorporated herein by reference.

The RF source 106, processor 102, and sensors 120 form part of anegative feedback system for regulating RF power output to theelectrodes. In the closed loop negative feedback system, the controllercompares the sensor data (such as temperature, impedance or currentdata) with reference data stored in memory to generate an error signal.The processor interfaces with power supply controller 104 and RF source106 (which may be combined into one circuit) to supply varying amountsof energy and power to the electrodes in order to decrease the errorsignal. Specifically, the RF source 106 is regulated to decrease theerror signal.

Processes to determine whether the electrodes are in contact with tissueare preferably performed both before and during a coagulation procedure.These processes are discussed in Section IV below. Should the processordetermine that there is a lack of sufficient contact, the level of powerto the electrodes 115 will not be increased to the level required forablation and, if desired, an audible and/or visual alarm will beactivated by the processor 102 in order to inform the physician that theelectrodes should be repositioned.

Upon detection of suitable contact between the electrodes 115 andtissue, the processor 102 instructs the RF power supply 106 and/or RFpower supply controller 104 to supply power to the electrodes. In oneembodiment the initial power setting for the apparatus is at 100 W powerat about 500 kHz, with a maximum of about 150 W power.

When processor 102 senses that the electrodes 115 have lost contact withtissue or are no longer in close enough proximity for efficacioustreatment, power to the electrodes is reduced (but not completelycut-off). Power may either be reduced to all of the electrodes, or onlyto those electrodes that have lost contact with tissue. The processormay use either temperature or impedance measurements to make thisdetermination. Preferably, power is reduced by ramping down theamplitude of the RF energy source. Additionally, a visual or audiblealarm may be provided to inform the physician that contact has been lostand power reduced. Once it is determined that contact or close proximityhas been reestablished, the power level may be ramped back up.

II. Electrosurgical Probe

A preferred electrosurgical probe is shown by way of example in FIG. 2,and described in detail in the aforementioned U.S. patent applicationSer. Nos. 08/949,117 and 09/072,835. The probe 208 includes a shaft 210and a handle 212 towards the proximal section of the shaft. The shaft210 consists of a hypo-tube, which is preferably either rigid orrelatively stiff, and an outer polymer coating 216 over the hypo-tube.The handle 212 preferably consists of two molded handle halves and isalso provided with strain relief element 222. An operative element 220is provided on the distal section 214 of the shaft 210. In theillustrated embodiment, the operative element is in the form of aplurality of spaced electrodes 224, which are preferably either rigidring-shaped electrodes or flexible helical electrodes.

The probe 208 may be used in a conventional electrosurgical systemconfiguration, where power transmission from an RF generator (or otherenergy source) to the electrodes 224 is controlled by a footswitch.Nevertheless, and shown by way of example in FIG. 2, a manually operableremote control 213 having individual on-off switches 215 is preferablyprovided in conjunction with the probe 208. Similar switches may also beprovided on the console 103. A global on-off switch (such as afootswitch, a switch on the handle 212, or a switch on the remotecontrol 213) may also be provided to allow the physician using theapparatus to selectively enable and disable the supply of RF energy toelectrodes 224. The individual on-off switches 215 allow the physicianto selectively control the supply of power to individual electrodes. Theexemplary remote control 213, which has seven individual on-off switches215, is preferably used in conjunction with a probe having sevenelectrodes. If, for example, the physician intends to ablate tissue withonly three of the electrodes, then the three chosen electrodes may beenabled by way of the corresponding switches 215 prior to placing theglobal on-off switch in the ON position.

A plurality of indicator elements 217 are also provided on the exemplaryremote control 213, as shown in FIG. 2. Preferably, there is oneindicator element for each of the on-off switches 215. The indicatorelements may also be in the form of indicator lights. Sound-basedindications of the on-off status of the switches 215 may also be used.For example, a speaker on the handle or the power control apparatus maybe employed to periodically indicate which of the switches 215 are inthe ON position.

A plurality of temperature sensors (not shown in FIG. 2) may be locatedon, under, abutting the edges of, or in between, the electrode elementsin any of the exemplary devices disclosed herein. Additionally, areference temperature sensing element may be provided on the handle 212,the shaft 210, or within the power supply and control apparatus.

III. Power Control

A. General

FIG. 3 shows, in schematic form, an exemplary system 300 for applyingsoft tissue coagulating (or "ablating") energy by multiple emittersbased, at least in part, upon local temperature conditions sensed bymultiple sensing elements.

In FIG. 3, the multiple sensing elements comprise thermocouples 308,309, and 310 individually associated with the multiple emitters ofenergy, which comprise electrode regions 301, 302, and 303 (such asthose located on the distal portion of the probe shown in FIG. 2). Thesystem 300 also includes a common reference thermocouple 311 carried forexposure to the blood pool. The reference thermocouple can also belocated on the probe handle or within the power control hardware.Alternatively, other kinds of temperature sensing elements can be used,like, for example, thermistors, fluoroptic sensors, and resistivetemperature sensors, in which case the reference thermocouple 311 wouldtypically not be required.

The system 300 further includes an indifferent electrode 319 foroperation in uni-polar mode.

The electrode regions 301, 302, 303 can comprise the rigid electrodesegments previously described. Alternatively, the electrode regions 301,302, 303 can comprise a continuous or segmented flexible electrode ofwrapped wire or ribbon. It should be appreciated that the system 300 canbe used in association with any energy emitting element that employsmultiple, independently actuated emitting elements.

The system 300 includes a source of energy 317, such as the RF energysources described above with reference to FIGS. 1 and 2. The source 317is connected (through a conventional isolated output stage 316) to anarray of power switches 314, one for each electrode region 301, 302, and303. A connector 312 (carried by the probe handle) electrically coupleseach electrode region 301, 303, 303 to its own power switch 314 and toother parts of the system 300.

The system 300 also includes a microcontroller 331 coupled via aninterface 330 to each power switch 314. The microcontroller 331, whichpreferably corresponds to the processor 102 shown in FIG. 1, turns agiven power switch 314 on or off to deliver RF power from the source 317individually to the electrode regions 301, 302, and 303. The deliveredRF energy flows from the respective electrode region 301, 302, and 303,through tissue, to the indifferent electrode 319, which is connected tothe return path of the isolated output stage 316.

The power switch 314 and interface 330 configuration can vary accordingto the type of energy being applied. FIG. 4 shows a representativeimplementation for applying RF energy.

In this implementation, each power switch 314 includes an N-MOS powertransistor 335 and a P-MOS power transistor 336 coupled in between therespective electrode region 301, 302, and 303 and the isolated outputstage 316 of the power source 317.

A diode 333 conveys the positive phase of RF energy to the electroderegion. A diode 334 conveys the negative phase of the RF energy to theelectrode region. Resistors 337 and 338 bias the N-MOS and P-MOS powertransistors 335 and 336 in conventional fashion.

The interface 330 for each power switch 314 includes two NPN transistors339 and 340. The emitter of the NPN transistor 339 is coupled to thegate of the N-MOS power transistor 335. The collector of the NPNtransistor 340 is coupled to the gate of the P-MOS power transistor 380.

The interface for each power switch 314 also includes a control bus 343coupled to the microcontroller 331. The control bus 343 connects eachpower switch 314 to digital ground (DGND) of the microcontroller 331.The control bus 343 also includes a (+) power line (+5V) connected tothe collector of the NPN transistor 339 and a (-) power line (-5V)connected to the emitter of the NPN interface transistor 340.

The control bus 343 for each power switch 314 further includes anE_(SEL) line. The base of the NPN transistor 339 is coupled to theE_(SEL) line of the control bus 343. The base of the NPN transistor 340is also coupled the E_(SEL) line of the control bus 343 via the Zenerdiode 341 and a resistor 332. E_(SEL) line connects to the cathode ofthe Zener diode 341 through the resistor 332. The Zener diode 341 isselected so that the NPN transistor 340 turns on when E_(SEL) exceedsabout 3 volts (which, for the particular embodiment shown, is logic 1).

It should be appreciated that the interface 330 can be designed tohandle other logic level standards. In the particular embodiment, it isdesigned to handle conventional TTL (transistor transfer logic) levels.

The microcontroller 331 sets E_(SEL) of the control bus 343 either atlogic 1 or at logic 0. At logic 1, the gate of the N-MOS transistor 335is connected to (+) 5 volt line through the NPN transistors 339.Similarly, the gate of the P-MOS transistor 336 is connected to the (-)5 volt line through the NPN transistor 340. This conditions the powertransistors 335 and 336 to conduct RF voltage from the source 317 to theassociated electrode region. The power switch 314 is "on."

When the microcontroller 331 sets E_(SEL) at logic 0, no current flowsthrough the NPN transistors 339 and 340. This conditions the powertransistors 335 and 336 to block the conduction of RF voltage to theassociated electrode region. The power switch 314 is "off."

The system 300 (see FIG. 3) further includes two analog multiplexers(MUX) 324 and 325. The multiplexers 324 and 325 receive voltage inputfrom each thermocouple 308, 309, 310, and 311. The microcontroller 331controls both multiplexers 324 and 325 to select voltage inputs from themultiple temperature sensing thermocouples 308, 309, 310, and 311.

The voltage inputs from the thermocouples 308, 309, 310, and 311 aresent to front end signal conditioning electronics. The inputs areamplified by differential amplifier 326, which reads the voltagedifferences between the copper wires of the thermocouples 308/309/310and the reference thermocouple 311. The voltage differences areconditioned by element 327 and converted to digital codes by theanalog-to-digital converter 328. The look-up table 329 converts thedigital codes to temperature codes. The temperature codes are read bythe microcontroller 331.

The microcontroller 331 compares the temperature codes for eachthermocouple 308, 309, and 310 to preselected criteria to generatefeedback signals. The preselected criteria are inputted through a userinterface 332. In addition to temperature, and as discussed below,criteria such as power, impedance and current may also be used togenerate feedback signals. These feedback signals control the interfacepower switches 314 via the interface 330, turning the electrodes 301,302, and 303 off and on.

The other multiplexer 325 connects the thermocouples 308, 309, 310, and311 selected by the microcontroller 331 to a temperature controller 315.The temperature controller 315 also includes front end signalconditioning electronics, as already described with reference toelements 326, 327, 328, and 329. These electronics convert the voltagedifferences between the copper wires of the thermocouples 308/309/310and the reference thermocouple 311 to temperature codes. The temperaturecodes are read by the controller and compared to preselected criteria togenerate feedback signals. These feedback signals control the amplitudeof the voltage (or current) generated by the source 317 for delivery tothe electrodes 301, 302, and 303.

Based upon the feedback signals of the microcontroller 331 and thetemperature controller 315, the system 300 distributes power to themultiple electrode regions 301, 302, and 303 to establish and maintain auniform distribution of temperatures along a lesion-forming element,such as the distal portion of the exemplary probe illustrated in FIG. 2.In this way, the system 300 obtains safe and efficacious lesionformation using multiple emitters of energy.

The system 300 can control the delivery of ablating energy in differentways. Representative modes will now be described.

B. Individual Amplitudes/Collective Duty Cycle

The electrode regions 301, 302, and 303 will be symbolically designatedE(J), where J represents a given electrode region (J=1 to N).

As before described, each electrode region E(J) has at least onetemperature sensing element 308, 309, and 310, which will be designatedS(J,K), where J represents the electrode region and K represents thenumber of temperature sensing elements on each electrode region (K=1 toM).

In this mode (see FIG. 5), the microcontroller 331 operates the powerswitch interface 330 to deliver RF power from the source 317 in multiplepulses of duty cycle 1/N.

With pulsed power delivery, the amount of power (P_(E)(J)) conveyed toeach individual electrode is as follows:

    P.sub.E(J) ˜AMP.sub.E(J).sup.2 ×DUTYCYCLE.sub.E(J)

where:

AMP_(E)(J) is the amplitude of the RF voltage conveyed to the electroderegion E(J), and

DUTYCYCLE_(E)(J) is the duty cycle of the pulse, expressed as follows:

    DUTYCYCLE.sub.E(J) =TON.sub.E(J) /[TON.sub.E(J) +TOFF.sub.E(J) ]

where:

TON_(E)(J) is the time that the electrode region E(J) emits energyduring each pulse period,

TOFF_(E)(J) is the time that the electrode region E(J) does not emitenergy during each pulse period.

The expression TON_(E)(J) +TOFF_(E)(J) represents the period of thepulse for each electrode region E(J).

In this mode, the microcontroller 331 collectively establishes dutycycle (DUTYCYCLE_(E)(J)) of 1/N for each electrode region (N being equalto the number of electrode regions).

The microcontroller 331 may sequence successive power pulses to adjacentelectrode regions so that the end of the duty cycle for the precedingpulse overlaps slightly with the beginning of the duty cycle for thenext pulse. This overlap in pulse duty cycles assures that the source317 applies power continuously, with no periods of interruption causedby open circuits during pulse switching between successive electroderegions.

In this mode, the temperature controller 315 makes individualadjustments to the amplitude of the RF voltage for each electrode region(AMP_(E)(J)), thereby individually changing the power P_(E)(J) of energyconveyed during the duty cycle to each electrode region, as controlledby the microcontroller 331.

In this mode, the microcontroller 331 cycles in successive dataacquisition sample periods. During each sample period, themicrocontroller 331 selects individual sensors S(J,K), and voltagedifferences are read by the controller 315 (through MUX 325) andconverted to temperature codes TEMP(J).

When there is more than one sensing element associated with a givenelectrode region, the controller 315 registers all sensed temperaturesfor the given electrode region and selects among these the highestsensed temperature, which constitutes TEMP(J).

In this mode, the controller 315 compares the temperature TEMP(J)locally sensed at each electrode E(J) during each data acquisitionperiod to a set point temperature TEMP_(SET) established by thephysician. Based upon this comparison, the controller 315 varies theamplitude AMP_(E)(J) of the RF voltage delivered to the electrode regionE(J), while the microcontroller 331 maintains the DUTYCYCLE_(E)(J) forthat electrode region and all other electrode regions, to establish andmaintain TEMP(J) at the set point temperature TEMP_(SET).

The set point temperature TEMP_(SET) can vary according to the judgmentof the physician and empirical data. A representative set pointtemperature for cardiac ablation is believed to lie in the range of 40°C. to 95° C., with 70° C. being a representative preferred value.

The manner in which the controller 315 governs AMP_(E)(J) canincorporate proportional control methods, proportional integralderivative (PID) control methods, or fuzzy logic control methods.

For example, using proportional control methods, if the temperaturesensed by the first sensing element TEMP(1)>TEMP_(SET), the controlsignal generated by the controller 315 individually reduces theamplitude AMP_(E)(1) of the RF voltage applied to the first electroderegion E(1), while the microcontroller 331 keeps the collective dutycycle DUTYCYCLE_(E)(1) for the first electrode region E(1) the same. Ifthe temperature sensed by the second sensing element TEMP(2)<TEMP_(SET),the control signal of the controller 315 increases the amplitudeAMP_(E)(2) of the pulse applied to the second electrode region E(2),while the microcontroller 331 keeps the collective duty cycleDUTYCYCLE_(E)(2) for the second electrode region E(2) the same asDUTYCYCLE_(E)(1), and so on. If the temperature sensed by a givensensing element is at the set point temperature TEMP_(SET), no change inRF voltage amplitude is made for the associated electrode region.

The controller 315 continuously processes voltage difference inputsduring successive data acquisition periods to individually adjustAMP_(E)(J) at each electrode region E(J), while the microcontroller 331keeps the collective duty cycle the same for all electrode regions E(J).In this way, the mode maintains a desired uniformity of temperaturealong the length of the distal portion of the exemplary probe shown inFIG. 2 or other lesion-forming element.

Using a proportional integral differential (PID) control technique, thecontroller 315 takes into account not only instantaneous changes thatoccur in a given sample period, but also changes that have occurred inprevious sample periods and the rate at which these changes are varyingover time. Thus, using a PID control technique, the controller 315 willrespond differently to a given proportionally large instantaneousdifference between TEMP (J) and TEMP_(SET), depending upon whether thedifference is getting larger or smaller, compared to previousinstantaneous differences, and whether the rate at which the differenceis changing since previous sample periods is increasing or decreasing.

C. Collective Amplitude/individual Duty Cycles

In this feedback mode (see FIG. 6), the controller 315 governs thesource 317 to collectively control the RF voltage amplitude AMP_(E)(J)for all electrode regions based upon the lowest local temperature sensedTEMP_(SMIN). At the same time, in this feedback mode, themicrocontroller 331 individually alters the power conveyed to theelectrode regions where temperatures greater than TEMP_(SMIN) aresensed, by adjusting the duty cycle DUTYCYCLE_(E)(J) of these electroderegions.

In this mode, as in the previous mode, the microcontroller 331 separatesthe power into multiple pulses. Initially, each pulse has the same dutycycle (DUTYCYCLE_(E)(J) of 1/N. As in the previous mode, the applicationof successive RF pulses to adjacent electrode regions may be timed tooverlap so that the source 317 applies power continuously to theelectrode regions E(J).

The controller 315 cycles in successive data acquisition periods tosequentially read the temperature sensed by each sensing elementTEMP(J). When there are multiple sensing elements associated with eachelectrode region, the controller 315 registers all sensed temperaturesfor the particular electrode and selects among these the highest sensedtemperature, which is TEMP(J).

In this mode, the controller 315 compares, during each data acquisitionperiod, the individual temperatures sensed TEMP(J) to the set pointtemperature TEMP_(SET). The controller 315 also selects the lowestsensed temperature TEMP_(SMIN). The controller 315 adjusts AMP_(E)(J) tomaintain TEMP_(SMIN>>) TEMP_(SET), using proportional, PID, or fuzzylogic control techniques. At the same time, the microcontroller 331adjusts DUTYCYCLE_(E)(J) of the electrode regions whereTEMP(J)>TEMP_(SMIN) to maintain TEMP(J)>> TEMP_(SET).

For example, using only proportional control techniques, if TEMP_(SMIN)<TEMP_(SET), the controller 315 collectively increases the amplitude ofthe RF voltage of all electrode regions, based upon the differencebetween TEMP_(SMIN) and TEMP_(SET) (ΔTEMP_(SMIN/SET)), until TEMP_(SMIN)>TEMP_(SET).

During this time (when TEMP_(SMIN) remains below TEMP_(SET)), themicrocontroller 331 also controls the application of power to the otherelectrode regions E(J) where the local sensed temperature TEMP(J) isabove TEMP_(SMIN), as follows:

(i) if TEMP(J)<TEMP_(SET), the microcontroller 331 increases the dutycycle of the power applied to the electrode region E(J) at the RFvoltage amplitude established by ΔTEMP_(SMIN/SET).

(ii) if TEMP(J)>TEMP_(SET), the microcontroller 331 decreases the dutycycle of the power applied to the electrode region E(J) at the RFvoltage amplitude established by ΔTEMP_(SMIN/SET).

(iii) if TEMP(J)_(S)(N) =TEMP_(SET), the microcontroller 331 maintainsthe duty cycle for the given electrode region E(N) at the RF voltageamplitude established by ΔTEMP_(SMIN/SET).

When TEMP_(SMIN) =TEMP_(SET), the controller 315 collectively reducesthe RF voltage amplitude delivered to all electrode regions. WhenTEMP_(SMIN) =TEMP_(SET), the controller 315 collectively maintains thethen-established RF voltage amplitude delivered to all electroderegions.

D. Temperature Control with Hysteresis

In this mode (see FIG. 7), as in the previous modes, the system 300cycles in successive data acquisition periods to sequentially registerthe temperature sensed by the sensing elements TEMP(J) for the electroderegions E(J). As before, when there are multiple sensing elementsassociated with each electrode region, the system 300 registers allsensed temperatures for the particular electrode region and selectsamong these the highest sensed temperature, which becomes TEMP(J).

In this mode, the microcontroller 331 compares the temperature sensedlocally at each electrode region TEMP(J) during each data acquisitionperiod to high and low threshold temperatures TEMP_(HITHRESH) andTEMP_(LOWTHRESH), where

    TEMP.sub.HITHRESH =TEMP.sub.SET +INCR

    TEMP.sub.LOWTHRESH =TEMP.sub.SET -INCR

where

TEMP_(SET) is the set point temperature, and

INCR is a preselected increment.

When operated in this mode, the microcontroller 331 operates the powerswitch interface 330 to turn a given electrode region E(J) off when thelocal temperature sensed at that electrode regionTEMP(J)>TEMP_(HITHRESH). The microcontroller 331 keeps the electroderegion turned off until the locally sensed temperature TEMP(J) dropsbelow TEMP_(LOWTHRESH). The microcontroller 331 turns a given electroderegion E(J) on and supplies power at a selected voltage amplitude whenthe local temperature sensed at that electrode regionTEMP(J)<TEMP_(LOWTHRESH).

The values for TEMP_(SET) and INCR can vary according to the judgment ofthe physician and empirical data. As before stated, a representativevalue for TEMP_(SET) is believed to lie in the range of 40° C. and 95°C., with a preferred value of 70° C. A representative value of INCR isbelieved to lie in the range of 2° C. to 8° C., with a preferredrepresentative value of around 5° C.

In this implementation, the controller 315 establishes a constant RFvoltage amplitude sufficiently high to maintain the desired temperatureconditions during hysteresis. Alternatively, the controller 315 can havethe capability to adjust voltage should the coolest sensed temperatureTEMP_(SMIN) decrease below a selected lower limit belowTEMP_(LOWTHRESH), or should the longest duty cycle exceed apredetermined value. It should be appreciated that there are other waysof adjusting and maintaining the amplitude while the hysteresis controlmethod is carried out.

E. Differential Temperature Disabling

In this mode (see FIG. 8), the temperature controller 315 selects at theend of each data acquisition phase the sensed temperature that is thegreatest for that phase (TEMP_(SMAX)). The temperature controller 315also selects for that phase the sensed temperature that is the lowest(TEMP_(SMIN)).

The controller 315 compares the selected hottest sensed temperatureTEMP_(SMAX) to a selected high set point temperature TEMP_(HISET). Thecomparison generates a control signal that collectively adjusts theamplitude of the RF voltage for all electrodes using proportional, PID,or fuzzy logic control techniques.

In a proportion control implementation scheme:

(i) If TEMP_(SMAX) >TEMP_(HISET), the control signal collectivelydecreases the amplitude of the RF voltage delivered to all electroderegions;

(ii) If TEMP_(SMAX) <TEMP_(HISET), the control signal collectivelyincreases the amplitude of the RF voltage delivered to all electroderegions;

(iii) If TEMP_(SMAX) =TEMP_(HISET), no change in the amplitude of the RFvoltage delivered to all electrode regions is made.

It should be appreciated that the temperature controller 315 can selectfor amplitude control purposes any one of the sensed temperaturesTEMP_(SMAX), TEMP_(SMIN), or temperatures in between, and compare thistemperature condition to a preselected temperature condition.

Working in tandem with the amplitude control function of the temperaturecontroller 315, the microcontroller 331 governs the delivery of power tothe electrode regions based upon difference between a given localtemperature TEMP (J) and TEMP_(SMIN). This implementation computes thedifference between local sensed temperature TEMP(J) and TEMP_(SMIN) andcompares this difference to a selected set point temperature differenceD TEMP_(SET). The comparison generates a control signal that governs thedelivery of power to the electrode regions.

If the local sensed temperature TEMP(J) for a given electrode regionE(J) exceeds the lowest sensed temperature TEMP_(SMIN) by as much as ormore than Δ TEMP_(SET) (that is, if TEMP(J)-TEMP_(SMIN) >ΔTEMP_(SET)),the microcontroller 331 turns the given electrode region E(J) off. Themicrocontroller 331 turns the given electrode E(J) back on whenTEMP(J)-TEMP_(SMIN) <ΔTEMP_(SET).

Alternatively (see FIG. 9), instead of comparing TEMP(J) andTEMP_(SMIN), the microcontroller 331 can compare TEMP_(SMAX) andTEMP_(SMIN). When the difference between TEMP_(SMAX) and TEMP_(SMIN)equals or exceeds a predetermined amount ΔTEMP_(SET), the controller 331turns all electrode regions off, except the electrode region whereTEMP_(SMIN) exists. The controller 331 turns these electrode regionsback on when the temperature difference between TEMP_(SMAX) andTEMP_(SMIN) is less than ΔTEMP_(SET).

Some of the above-described temperature-based control schemes alterpower by adjusting the amplitude of the RF voltage. It should beappreciated that, alternatively, power can be altered by the adjustingthe amplitude of RF current. Therefore, the quantity AMP_(E)(J) used inthis Specification can mean either RF voltage amplitude or RF currentamplitude.

F. Deriving Predicted Hottest Temperature

As previously described, a given electrode region can have more than onetemperature sensing element associated with it. In the previouslydescribed control modes, the controller 315 registers all sensedtemperatures for the given electrode region and selects among these thehighest sensed temperature, which constitutes TEMP(J). There arealternative ways of making this selection. One way is to derive thepredicted hottest temperature.

Because of the heat exchange between the tissue and the electroderegion, the temperature sensing elements may not measure exactly themaximum temperature at the region. This is because the region of hottesttemperature occurs beneath the surface of the tissue at a depth of about0.5 to 2.0 mm from where the energy emitting electrode region (and theassociated sensing element) contacts the tissue. If the power is appliedto heat the tissue too quickly, the actual maximum tissue temperature inthis subsurface region may exceed 100° C. and lead to tissue desiccationand/or micro-explosion.

FIG. 10 shows an implementation of a neural network predictor 400, whichreceives as input the temperatures sensed by multiple sensing elementsS(J,K) at each electrode region, where J represents a given electroderegion (J=1 to N) and K represents the number of temperature sensingelements on each electrode region (K=1 to M). The predictor 400 outputsa predicted temperature of the hottest tissue region T_(MAXPRED) (t).The controller 315 and microcontroller 331 derive the amplitude and dutycycle control signals based upon T_(MAXPRED) (t), in the same mannersalready described using TEMP(J).

The predictor 400 uses a two-layer neural network, although more hiddenlayers could be used. As shown in FIG. 10, the predictor 400 includes afirst and second hidden layers and four neurons, designated N.sub.(L,X),where L identifies the layer 1 or 2 and X identifies a neuron on thatlayer. The first layer (L=1) has three neurons (X=1 to 3), as followsN.sub.(1,1) ; N.sub.(1,2) ; and N.sub.(1,3). The second layer (L=2)comprising one output neuron (X=1), designated N.sub.(2,1).

Temperature readings from the multiple sensing elements, only two ofwhich--TS1(n) and TS2(n)--are shown for purposes of illustration, areweighed and inputted to each neuron N.sub.(1,1) ; N.sub.(1,2) ; andN.sub.(1,3) of the first layer. FIG. 10 represents the weights asW^(L).sub.(k,N), where L=1; k is the input sensor order; and N is theinput neuron number 1, 2, or 3 of the first layer.

The output neuron N.sub.(2,1) of the second layer receives as inputs theweighted outputs of the neurons N.sub.(1,1) ; N.sub.(1,2) ; andN.sub.(1,3). FIG. 10 represents the output weights as W^(L).sub.(O,X),where L=2; O is the output neuron 1, 2, or 3 of the first layer; and Xis the input neuron number of the second layer. Based upon theseweighted inputs, the output neuron N.sub.(2,1) predicts T_(MAXPRED) (t).Alternatively, a sequence of past reading samples from each sensor couldbe used as input. By doing this, a history term would contribute to theprediction of the hottest tissue temperature.

The predictor 400 must be trained on a known set of data containing thetemperature of the sensing elements TS1 and TS2 and the temperature ofthe hottest region, which have been previously acquired experimentally.For example, using a back-propagation model, the predictor 400 can betrained to predict the known hottest temperature of the data set withthe least mean square error. Once the training phase is completed thepredictor 400 can be used to predict T_(MAXPRED) (t).

The predicted tissue temperature can also be used to adjust thetemperature set curve. For example, if the predictor 400 predicts arelatively high tissue temperature, then the temperature set curve canbe adjusted downwardly and vice versa. The duration of an ablationprocedure can also be adjusted based on predicted tissue temperature.

Other types of data processing techniques can be used to deriveT_(MAXPRED) (t). See, e.g., co-pending patent application Ser. No.08/801,484, filed Feb. 18, 1997, and entitled "Tissue Heating andAblation Systems and Methods Using Predicted Temperature for Monitoringand Control."

In addition to being used in the previously described temperaturecontrol systems, the predicted temperature of the hottest tissue regionT_(MAXPRED) (t) can be used in other power control systems or to adjustother power control parameters.

For example, T_(MAXPRED) (t) may be used to adjust a temperature setpoint curve upwardly or downwardly. Either the entire curve, or just aportion thereof, may be adjusted. One example of such a temperature setpoint curve is the variable temperature set point curve discussed inSection V below. Another example is disclosed in U.S. Pat. No.5,755,715, entitled "Tissue Heating and Ablation Systems and MethodsUsing Time-Variable Set Point Curves for Monitoring and Control."Adjusting a temperature set point curve upwardly will occur whenT_(MAXPRED) (t) is lower than the assumed maximum temperature thatformed the basis for the set point curve. Such an adjustment willdecrease the duration of the coagulation procedure, as compared toduration without the adjustment. The increase in speed is especiallyuseful when the patient is on cardiopulmonary bypass. Conversely, whenT_(MAXPRED) (t) is higher than the assumed maximum temperature, thetemperature set point curve will be adjusted downwardly, therebyreducing the likelihood of undesired damage to ancillary tissue as wellas charring or popping of the targeted tissue.

T_(MAXPRED) (t) may also be used to increase or decrease the duration ofa coagulation procedure. Adjusting a duration upwardly will occur whenT_(MAXPRED) (t) is lower than the assumed maximum temperature thatformed the basis for the original duration estimation. This will insurethat a therapeutic lesion will be formed. The duration of a coagulationprocedure will be reduced when T_(MAXPRED) (t) is higher than theassumed maximum temperature, thereby reducing the length of time that apatient is on bypass as well as the likelihood of undesired tissuedamage.

IV. Monitoring Tissue Contact, Coagulation Efficacy and Tissue Type

In addition to maintaining a set temperature, electrode-tissue contactmay also be monitored using the same process of sensing a variable (suchas temperature, impedance or current), comparing it with a referencesignal, and generating an error signal that may be used to control poweroutput and audio/visual indicators on the console. The sensed data mayalso be used to make other determinations, such as tissue viability andtissue type. Preferably, all three of the determinations (i.e. contact,viability and type) will be made at at least one point during thecoagulation procedure.

Turning first to impedance, the graph illustrated in FIG. 11(a) isrepresentative of the impedance magnitude versus frequency curve forviable tissue, while the graph illustrated in FIG. 11(b) isrepresentative of the curve for coagulated tissue or an instance wherethere is insufficient contact between the electrodes and tissue and theelectrode is in blood, and the graph illustrated in FIG. 11(c) isrepresentative of the curve when the electrodes are in air.

For viable tissue, there is a generally downward sloping curve asfrequency increases, having a relative maximum point MAX1 at frequencyf1 (about 1 kHz) and a relative minimum point MIN2 at frequency f2(about 100 kHz). Under normal circumstances, the curve flattens out, asshown in FIG. 11(b), when the tissue is coagulated. However, therelatively flat curve is also indicative of a situation where theelectrode tissue contact has been lost and the sensing electrodes aresurrounded by blood. The processor could be used to take derivatives ofthe impedance curve to ascertain whether a change in the curve hasoccurred. Alternatively, the processor may be used to compare thedifference between the values of MAX1 and MIN2 (or the MAX1/MIN2 ratio)with sensed values at the same frequencies until the difference (orratio) is smaller than a predetermined threshold value, which indicatescoagulation has been achieved. In addition, or alternatively, a table ofimpedance versus frequency curve points may be stored in memory and usedfor comparison purposes.

As noted above, one method of measuring impedance involves measuring thevoltage and current and dividing the measured voltage by the measuredcurrent. When using a probe having four spaced electrodes E₁ -E₄, thetechnique illustrated in FIG. 11(d) may be used.

The particular f1 and f2 values and MAX1 and MIN2 values can varyaccording to tissue characteristics, as can the f1/f2 ratio and theMAX1/MIN2 ratio. For example, cardiac tissue and liver tissue may havedifferent f1, f2, MAX1 or MIN2 values. This phenomenon is discussed inmore detail in Rabbat A: Tissue resistivity. In Webster JG, ed:Electrical Impedance Tomography. Adam Hilger, Bristol, 1990. As such,impedance measurements may be used to determine tissue type. The tissuetype determination may in turn be used both before and during acoagulation procedure to insure that the intended type of tissue will beand is being coagulated.

Additionally, if the impedance level exceeds 300 ohms, which isindicative of a situation where the electrodes are in air, then theenergy level can be reduced to 5 V for a period of up to 10 seconds. Thereduction can be either an immediate reduction or a ramping reduction.If the impedance level continues to exceed 300 ohms at the end of theperiod, then energy delivery will be stopped.

With respect to current, it can be used to both measure tissue contactand to prevent tissue charring and the formation of coagulum. For agiven input maximum power level, there is a current level which, ifreached, is either indicative of poor contact or of tissue charring andcoagulum formation. For example, using 12.5 mm electrodes and a setpower level of 70W, 0.9A is a preferable current limit, while 0.7A is apreferable limit when 6 mm electrodes are used with the same powersetting.

Turning to temperature, prior to the coagulation procedure, a relativelysmall amount of energy (preferably about 20 V for 3 seconds) may bedelivered to the tissue prior to the application of the larger amount ofpower necessary to coagulate tissue. The relatively small amount ofpower will increase the temperature of the tissue by a predeterminedamount when the electrodes are in efficacious contact with the tissue.By comparing the sensed temperature rise to the expected temperaturerise associated with efficacious contact (greater than or equal to about1° C. per second), the processor can determine whether there issufficient contact for the procedure to continue.

A temperature-based process may also be used to ascertain when tissuehas been properly coagulated. Here, the controller monitors the sensedtissue temperature profile history (i.e. temperature over time) for agiven power level. The temperature profile history is compared to apredetermined history for that power level stored in memory todetermined when the coagulation process has ended. The controller willthen cause the power circuit to end the delivery RF energy to theablation electrode and, if desired, cause the console to provide anaudio and/or visual indication that coagulation is complete.

As described above, certain aspects of the disclosed embodimentsillustrated in FIGS. 1-10 combine negative feedback techniques with anautomatic process controller. The controller (element 102 in FIG. 1 andelement 331 in FIG. 3) compares sensed controlled variable data (such astemperature, impedance or current) to reference signal data, that iseither input by the user or stored in memory, to generate an errorsignal. The error signal is used as input to the process controller thatgenerates correcting influence on the feedback network circuit. Thecorrecting influence can be used to affect the RF pulse trainillustrated in FIG. 1. In the case of temperature feedback, the amountof energy delivered to the probe can be varied in order to maintain atemperature set point. In the cases of temperature, impedance andcurrent, power can be ramped down to a predetermined level during acoagulation procedure when there is an indication that tissue contact isinsufficient.

As discussed previously, the voltage output to the electrodes may bedecreased from a higher level to a lower level upon a loss oftissue-electrode contact, as determined by the controller based on anerror signal associated with measured temperature or impedance. The rateof RF voltage increase may also be limited upon resumption ofcoagulation to avoid tissue damage caused by overheating due totransient overshoot of high voltages. For example, voltage may bereduced to 5-20V if the measured temperature is more than 10° C. higherthan the set temperature to avoid overheating or when there has been aloss of tissue-electrode contact. Once the overheating has beeneliminated or contact has been reestablished, the power may be rampedback up to the level necessary to achieve the set temperature over aperiod of about 5 seconds.

V. Variable Temperature Set Point

In accordance with one of the present inventions, a variable temperatureset point may be employed in a coagulation procedure. In other words,instead of immediately ramping up to the input desired maximum tissuetemperature and maintaining that temperature, the controller first rampstissue temperature up to at least one other temperature which is lowerthan the set maximum temperature. The controller then maintains thattemperature for a predetermined period prior to increasing tissuetemperature to the desired maximum temperature. This may be accomplishedby, for example, using a PID temperature control algorithm with atemperature setpoint that varies over time, also referred to as avariable temperature set point curve.

The temperature at which tissue is maintained prior to ramping up to theinput maximum temperature should be at least sufficient to desiccatetissue and is preferably sufficient to create a transmural lesion in arelatively thin tissue structure. The maximum temperature is preferablya temperature that will create a transmural lesion in a relatively thickstructure. Thus, in those instances where the tissue structure turns outto be relatively thin, a transmural lesion may be completed (and powerdelivery stopped) before the temperature reaches the maximumtemperature. The determination of lesion completion may be made throughvisual inspection or through the use of the techniques described above.Should the tissue structure turn out to be relatively thick, temperaturewill continue to ramp up to the maximum temperature.

As shown by way of example in FIG. 13, one embodiment of the inventionincludes a temperature control algorithm wherein temperature ismaintained at two different temperatures prior to reaching the settemperature T_(SET). In the illustrated embodiment, the set point curveis based on a coagulation procedure where T_(SET) is 70° C. and thetotal energy application time is 60 seconds. Both of these variables maybe input by the physician. The controller initially sets the temperaturesetpoint at a first temperature T₁ for about 10 to 15 seconds.Temperature T₁ is about 10° C. less than T_(SET). The controller thenincreases the temperature setpoint to a second temperature T₂ for about10 seconds. Temperature T₂ is about 5° C. less than T_(SET). Finally,the temperature setpoint is increased to the input temperature setpointT_(SET) for the remainder of the coagulation procedure.

The exemplary temperature set point curve described in the precedingparagraph is especially useful when creating a transmural lesion on theepicardial surface. Of course, other curves can be used in othersituations. In endocardial applications where a transmural lesion isdesired, for example, a temperature setpoint T_(SET) of 70° C. may benecessary. The temperature set point curve used here preferably startsat a first temperature T₁ (about 60° C.) for about 10 to 15 seconds.Thereafter, the temperature setpoint increases at a rate of betweenabout 1 and 2° C. per second until it reaches the input setpoint of 70°C. Other useful applications of the variable temperature setpointinclude coagulation procedures where expandable (or "balloon")electrodes and other relatively large electrodes are used. Here, asetpoint curve wherein a first temperature setpoint T₁ (about 60° C.)was maintained for about 10 seconds and then increased at a rate ofbetween about 1 and 2° C. per second until reaching an input setpoint ofabout 85° C. was found to be useful.

A variety of variable temperature setpoint curves that are specificallydesigned for a variety of procedures may be stored in the controllermemory. Alternatively, or in addition, the controller may be providedwith a program that generates a variable temperature setpoint curvebased on input parameters such as maximum temperature, total energyapplication time, and type of procedure.

VI. User Interface System

One advantageous feature of the present user interface is that it willprovide an indication that a therapeutic lesion has been successfullyformed on the intended type of tissue. For example, some lesions will beformed based on information input by the physician via the userinterface. The information may include lesion type, tissue type and typeof probe (including electrode configuration). Probe type may also beautomatically input by providing a device on the probe which isindicative of probe type in, for example, the manner disclosed in U.S.Pat. No. 5,743,903, which is incorporated herein by reference. Thesystem controller will use these parameters to, for example, select atime variable set curve, the appropriate power level and a total energyapplication time. Of course, time, temperature, power level and otherparameters can also input manually.

At the end of the input or automatically selected time period, the userinterface will either instruct the operator to discontinue powerapplication or simply indicate that power has been discontinued. Thesame operations may be performed in those instances where the controllerdetermines that a therapeutic lesion has been formed based on, forexample, tissue impedance measurements. Should the physician reach thepoint where there is an instruction to discontinue power (or power isautomatically discontinued) without an error signal (such as loss ofcontact) he or she will know that a therapeutic lesion has been formed.Alternatively, an audible or visual indication may be provided when atherapeutic lesion has been successfully formed.

In accordance with one embodiment of the invention, software drives theprocessor 102 to operate the coagulation processes described above. Suchsoftware will also drive operation of a user interface, such as theconsole 103 illustrated in FIGS. 1 and 2, which provides informationobtained before and during the coagulation procedure.

A flow chart illustrating the steps performed by the exemplary softwareis illustrated in FIG. 12. The software may be written in any standardindustry software language, such as the C or C++ languages. It iscontemplated that the software is executable code residing in RAM, ormay be part of a dedicated processor existing as firmware, ROM, EPROM orthe like. The steps listed in the program correspond to, inter alia,function calls, modules, subroutines, classes, and/or lines ofexecutable code in a machine readable program operating a generalpurpose microprocessor and, therefore, constitute apparatus forexecuting such steps in addition to methods of operation.

In step 501, the software program performs a power-on self test. Here,the program checks to see whether the supply voltages are within thespecified range, whether the front panel console and remote LEDs arefunctional, etc. In step 503, the initial selected values are displayedby the console. Such values may include a set tissue temperature with,for example, an initial default value of 70° C. and measured values fromthe sensors. Step 505 is the start of the switch checking process. Instep 507, the application of RF energy is started if the RF energy ONswitch is depressed. Otherwise, the power up/down switch is checked(step 509), and if that switch is depressed, the upper power limitallowed for the power supply is adjusted as necessary in step 511. Ifthe power up/down switch is not depressed, the temperature up/downswitch is checked (step 513) and the temperature setpoint is adjusted asnecessary in step 515. If the temperature up/down switch is notdepressed, the process loops back to step 505.

In step 517, which follows a RF energy ON finding in step 507, theablation process begins. Here, an "RF energy ON" signal is provided bythe console and an audible tone emitted. The global power ON/OFFswitch(es) are checked in step 519, such as a foot pedal switch orswitch on a remote control device. In step 521, the processor checks forelectrode contact verification between the electrodes and tissue and, ifthere is contact (step 523), proceeds to step 525. Otherwise, thecoagulation process is either stopped by ramping down power to apredetermined level, disabling a selected electrode or, if the processhas yet to begin, simply not started. Audio and/or visual signalsconcerning the lack of suitable contact are then activated (step 527)and the software returns to step 503.

If the RF power ON switch is still activated in step 525, the powerincrement UP/DOWN switch is checked (step 529). Otherwise, the programreturns to step 527. If the power increment UP/DOWN switch is depressed,step 531 is pursued to adjust the power limit, while continuing thecoagulation process, and the return path 533 is taken to step 519. Ifthe power increment UP/DOWN switch has not been depressed, thetemperature UP/DOWN switch is checked (step 535). The temperaturesetpoint is adjusted in step 537, while continuing the coagulationprocess, and then returning to step 519.

Additional and optional steps may be added to the software after step535. [Note steps 540 and 545.] Such additional steps may, for example,include determining whether or not a probe has been attached to thepower supply and control apparatus, determining the elapsed time ofcoagulation, determining the power outputted. Other additional stepsinclude providing an audible tone at a predetermined interval, such asevery 30 seconds, and displaying the probe type. Also, at each and everystep, the processor may refresh the console display with relevant datathat is displayed.

This specification discloses multiple electrode structures in thecontext of cardiac tissue coagulation because the structures are wellsuited for use in the field of cardiac treatment. Nevertheless, itshould be appreciated that the disclosed structures are applicable foruse in other applications. For example, various aspects of the inventionhave applications and procedures concerning other regions of the bodysuch as the prostate, brain, gall bladder and uterus.

Although the present invention has been described in terms of thepreferred embodiments above, numerous modifications and/or additions tothe above-described preferred embodiments would be readily apparent toone skilled in the art. It is intended that the scope of the presentinvention extends to all such modifications and/or additions and thatthe scope of the present invention is limited solely by the claims setforth below.

We claim:
 1. An apparatus for coagulating tissue, comprising:a supportelement; at least one operative element on the support element; and acontrol system, operably connected to the operative element, and adaptedto measure impedance and determine whether the operative element is incontact with tissue and whether tissue in proximity to the operativeelement is viable tissue or non-viable tissue, at least one of thedeterminations being based on an impedance versus frequency profile. 2.An apparatus as claimed in claim 1, wherein the contact determination isbased on the impedance versus frequency profile.
 3. An apparatus asclaimed in claim 1, wherein the viability determination is based on theimpedance versus frequency profile.
 4. An apparatus as claimed in claim1, wherein the operative element comprises an electrode.
 5. An apparatusas claimed in claim 1, wherein the control system is adapted to generateat least one of an audio alarm and a visual alarm in response to atleast one of a non-viable tissue determination and a loss of contactdetermination.
 6. An apparatus as claimed in claim 1, wherein thesupport element comprises a relatively stiff shaft.
 7. An apparatus asclaimed in claim 1, wherein the control system determines whether theoperative element is in contact with viable tissue prior to initiating acoagulation procedure.
 8. An apparatus for coagulating tissue,comprising:a support element; at least one operative element on thesupport element; and a control system, operably connected to theoperative element, that measures temperature at the operative elementand determines whether the operative element is in contact with tissueand whether tissue in proximity to the operative element is viabletissue or non-viable tissue, at least one of the determinations beingbased on the temperature measurement.
 9. An apparatus as claimed inclaim 8, wherein the operative element comprises an electrode.
 10. Anapparatus as claimed in claim 8, wherein the control system is adaptedto generate at least one of an audio alarm and a visual alarm inresponse to at least one of a non-viable tissue determination and a lossof contact determination.
 11. An apparatus as claimed in claim 8,wherein the support element comprises a relatively stiff shaft.
 12. Anapparatus as claimed in claim 8, wherein the control system determineswhether the operative element is in contact with viable tissue prior toinitiating a coagulation procedure.
 13. An apparatus for coagulatingtissue, comprising:a support element; at least one electrode on thesupport element; a source of tissue coagulating energy operablyconnected to the electrode and adapted to supply energy at a first levelsuitable for tissue coagulation and at a second, non-zero level lowerthan the first level; and a control system, operably connected to theelectrode, and adapted to determine whether the electrode is in contactwith tissue and whether tissue in proximity to the electrode is viabletissue or non-viable tissue and to reduce the energy level from thefirst level to the second level in response to the loss of contactdetermination.
 14. An apparatus as claimed in claim 13, wherein theenergy source comprises a radio frequency energy source.
 15. Anapparatus as claimed in claim 13, wherein the control system causes theenergy source to ramp down to the second level in response to the lossof contact determination.
 16. An apparatus as claimed in claim 13,wherein the control system causes the energy source to ramp up to thefirst level in response a determination that contact between the atleast one electrode and tissue has been reestablished.
 17. A method ofcoagulating tissue, comprising the steps of:attempting to place anoperative element in at least close proximity to tissue; performing afirst data sensing operation to sense impedance data at variousfrequencies; comparing the impedance data to an impedance versusfrequency profile to determine one of whether the electrode is incontact with tissue and whether tissue in proximity to the electrode isviable tissue or non-viable tissue; performing a second data sensingoperation; and using the data sensed during the second data sensingoperation to determine the other of whether the electrode is in contactwith tissue and whether tissue in proximity to the electrode is viabletissue or non-viable tissue.
 18. A method as claimed in claim 13,wherein the impedance data is used to determine whether the operativeelement is in contact with tissue.
 19. A method as claimed in claim 13,wherein the impedance data is used to determine whether the tissue isviable tissue or non-viable tissue.
 20. A method of coagulating tissue,comprising the steps of:attempting to place an operative element in atleast close proximity to tissue; performing a first data sensingoperation to sense temperature data; using the temperature data todetermine whether the tissue is viable tissue or non-viable tissue; andperforming a second data sensing operation to determine whether theoperative element is in contact with the tissue.
 21. An apparatus forcoagulating tissue, comprising:a support element; at least one tissueheating element on the support element; at least one temperature sensoron the support element; a source of tissue heating energy operablyconnected to the tissue heating element and adapted to supply energy ata first level suitable for tissue coagulation and at a second, non-zerolevel lower than the first level, the second level being suitable toheat tissue with a predetermined rate of temperature rise when thetissue heating element is in contact with the tissue; and a controlsystem, operably connected to the at least one temperature sensor, andadapted to determine whether the tissue heating element is in contactwith the tissue based on feedback from the temperature sensor.
 22. Anapparatus as claimed in claim 13, wherein the control system is adaptedto cause the energy source to provide energy to the at least one tissueheating element at the second level, receive a temperature signal fromthe at least one temperature sensor, and determine whether the tissueheating element is in contact with the tissue by determining whether thetemperature has reached the predetermined temperature.
 23. An apparatusas claimed in claim 22, wherein the control system is adapted to causethe energy source to provide energy at the first level in response adetermination that the tissue has reached the predetermined temperature.24. An apparatus as claimed in claim 22, wherein the control system willnot cause the energy source to provide energy at the first level until adetermination that the tissue has reached the predetermined temperature.25. An apparatus as claimed in claim 22, wherein the control system isadapted to generate at least one of an audio alarm and a visual alarm inresponse to a determination that tissue did not reach the predeterminedtemperature in response to an application of energy at the second level.26. An apparatus as claimed in claim 21, wherein the support elementcomprises a relatively stiff shaft.
 27. An apparatus as claimed in claim21, wherein the tissue heating element comprises an electrode.
 28. Anapparatus as claimed in claim 21, wherein the source of tissue heatingenergy comprises a source of RF energy.
 29. A method of coagulatingtissue, comprising the steps of:attempting to place a tissue heatingelement in contact with tissue; supplying the tissue heating elementwith energy at a first level insufficient to coagulate the tissue andsufficient to cause the tissue to rise to a predetermined temperaturelevel when the tissue heating element is in contact with the tissue;sensing the temperature of the tissue; and determining whether thetissue heating element is in contact with tissue based on the sensedtemperature of the tissue.
 30. A method as claimed in claim 29, furthercomprising the step of:supplying the tissue heating element with energyat a second level sufficient to coagulate tissue in response to adetermination that the tissue heating element is in contact with tissue.31. A method as claimed in claim 21, wherein the step of determiningwhether the tissue heating element is in contact with tissue comprisesdetermining whether the temperature of the tissue rises at apredetermined rate.
 32. An apparatus for coagulating tissue,comprising:a support element; at least one electrode on the supportelement; a source of tissue coagulating energy operably connected to theelectrode and adapted to supply energy at a first voltage suitable fortissue coagulation and at a second, non-zero voltage lower than thefirst voltage; and a control system, operably connected to energysource, and adapted to determine when there is a loss of contact betweenthe at least one electrode and tissue and to reduce the voltage levelfrom the first voltage to the second voltage in response to the loss ofcontact determination.
 33. An apparatus as claimed in claim 32, whereinthe control system causes the energy source to ramp down to the secondvoltage in response to the loss of contact determination.